JPS6049297B2 - optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an isolator in the light wave region, and particularly to an optical isolator suitable for optical fiber communications.

Recently, research and development of optical fiber communication has progressed rapidly and its practical application is progressing. However, it is important to improve the performance of various optical devices (optical fiber, coupling circuit between light source and optical fiber, etc.) used in optical fiber communication, especially insertion loss. With this decline, new problems have arisen.

In other words, in order for the reflected light from the end faces of various optical devices to return to the light source without much loss, if the light source is a laser oscillator, it will have an adverse effect on its operating characteristics, and in some cases may deteriorate the communication quality. The problem is that it causes extreme deterioration. Additionally, multiple reflections at the end faces of various optical devices cause echoes to be added to the transmitted signal. Deterioration of the operating characteristics of a laser oscillator due to feedback of reflected light has been observed in relation to the stability of self-mode locking, etc., and to prevent this deterioration, reflected light removal using a combination of a polarizing plate and an i-wavelength plate has been observed. circuits, optical isolators that combine polarizers, Faraday rotation elements, and analyzers, etc. have been used, but these circuits have not been able to satisfactorily solve the above problems associated with optical fiber communications. .
This is because these circuits are theoretically lossless for linearly polarized light components in a certain direction of forward incident light, but all linearly polarized light components orthogonal to this component will be lost. Optical fibers, on the other hand, cannot normally transmit light (while maintaining its polarized state), so connecting the above circuit behind an optical fiber will almost always result in
This is because a loss of about dB occurs, and the value of this loss changes depending on the polarization state. Sara' 1,
A reflected light removal circuit that combines a polarizing plate and a j-wavelength plate will detect the reverse direction only when the polarization state of the light that passes through this circuit in the forward direction remains unchanged and is reflected back as circularly polarized light. Since it can only sufficiently eliminate reflected light (therefore, this circuit cannot be called an optical isolator), it cannot sufficiently eliminate reflected light in the opposite direction, which is not necessarily circularly polarized light, that returns through the optical fiber. Nakatsuta.

An object of the present invention is to provide an optical isolator which is theoretically lossless for forward incident light regardless of its polarization state, and therefore has low loss and little polarization dependence.

According to this invention, the first birefringent element splits a forward incident light beam into two linearly polarized light beams according to its polarization, and the polarization direction of each linearly polarized light beam is set to M,
．． (2n1) right angle and (2
n±h) An irreversible polarization rotation element that rotates by a right angle (same order), and a second element that combines two linearly polarized light beams that have passed through this irreversible polarization rotation element using the orthogonality of the polarizations. An optical isolator including a birefringent element is obtained.
The first birefringent element of the present invention splits a forward incident light beam into two linearly polarized light beams according to its polarization, and each of the two branched linearly polarized light beams passes through an irreversible polarization rotation element. After that, they are synthesized by a second birefringent element. The polarization directions of the two linearly polarized light beams split by the first birefringent element are at right angles to each other, and this orthogonality does not change even if the two linearly polarized light beams pass through the irreversible polarization rotation element. The second birefringent element uses this orthogonality to convert the linearly polarized light beam into a linearly polarized light beam with no loss in principle.
It is synthesized. Next, if the output light beam of this optical isolator is reflected and returns with an arbitrary polarization state,
Furthermore, generally speaking, when considering the case where a light beam enters this optical isolator from the opposite direction, the light beam first passes through the second birefringent element in the opposite direction and is split into two linearly polarized light beams. By the way, since the birefringent element is reversible, these two branched linearly polarized light beams are
Two linearly polarized light beams in the forward direction, each with the same direction of polarization, and each with the same light beam. (but in the opposite direction) and enters the irreversible polarization rotation element from the reverse direction. The polarization directions of the two linearly polarized light beams entering the irreversible polarization rotation element are changed to the same direction by passing through this element.
2n1±↓) right angle and hiichi (2n shi closed) right angle (compound same order)
The difference in polarization direction with the forward linearly polarized light beam on the same optical path is 2 x (2r] 1 person) right angle = (4 m ±
1) Right angle and (4n±1) right angle (for forward and reverse direction light, the rotation direction of polarized light of the irreversible polarization rotation element as seen from the light traveling direction is opposite, so this element is When the light goes back and forth, the angle of polarization rotation seen from the forward direction doubles.) A difference between two right angles in polarization direction is the same as no difference. Therefore, the polarization direction of the backward linearly polarized light beam that has passed through the irreversible polarization rotation element is orthogonal to that of the forward linearly polarized light beam on the same optical path. Therefore, when a linearly polarized light beam like this) follows the same optical path as the forward linearly polarized light beam in the opposite direction and enters the first birefringent rotation element,
It is clear from the reversibility of this element that it is not possible to reverse the optical path of the forward light beam, but it exits through a different optical path. That is, the light beam in the opposite direction does not return along the optical path of the light beam in the forward direction, and therefore the function of an optical isolator is fulfilled. Taking a closer look at the loss of forward incident light in the optical isolator of this invention, the loss when the first birefringent element splits the incident light beam into two linearly polarized light beams orthogonal to each other is the principle. If the polarization state of the incident light beam changes, only the intensity ratio of the two branched linearly polarized light beams changes, and the sum of the intensities is, in principle, equal to the intensity of the incident light beam.

Since the irreversible polarization rotation element through which these two linearly polarized light beams pass next is also lossless in principle, the sum of the intensities of these two linearly polarized light beams that are incident on the second birefringent element is also The same can be said. The second birefringent element that combines these two linearly polarized light beams performs this combination using the orthogonality of polarization, so the loss at that time can be made lossless in principle, and the combined output light beam The strength of is 2 before composition.
The orthogonality of the polarizations of the two linearly polarized light beams results in (in principle) the sum of their intensities and thus equal to the intensity of the incident light beam. In other words, the optical isolator of the present invention has no loss in principle with respect to the forward incident light beam, and the loss does not change depending on the polarization state of the incident light beam in principle. In reality, the elements that make up this optical isolator have some loss, but since it is possible to use low-loss elements, it is possible to realize an optical isolator that has low loss and is almost independent of the polarization state of the incident light beam. can. Next, this invention will be explained with reference to the drawings.

FIG. 1 shows the most preferred embodiment of the invention (hereinafter referred to as the first
FIG.

The light emitted radially from the end face of the first optical fiber 1 is converted into a parallel light beam 101 by the pitched first converging transmitter 11 and enters the first birefringent crystal 21 (convergent Regarding the lens action of optical transmission bodies, etc.
7-816, Japanese Patent Publication No. 47-6547, etc., so I will not discuss it here). This light beam 101
The direction in which the robot moves is called the forward direction, and this direction is shown by a solid arrow in the figure (the same applies hereafter).

Also, dotted arrows indicate the opposite direction. As mentioned above, since the polarization state of the light passing through the optical fiber cannot be specified, the polarization state of the light beam 101 is arbitrary, and is represented by the polarization state diagram 151 in the figure (the same applies hereinafter). That is, Xyz
If the coordinate system is defined as shown in the figure, the polarization state of the light beam 101 can generally be expressed as a combination of the polarization component in the X direction and the polarization component in the y direction, so the polarization state diagram 151 shows these two polarization directions. (shown in terms of the direction of the electric field) is shown ignoring the intensity ratio and phase relationship of the two polarized light components. Hereinafter, in this type of polarization state diagram, solid arrows indicate the polarization direction of the forward light beam, and dotted arrows indicate the polarization direction of the reverse light beam. The first birefringent crystal 21 is made of calcite, and two parallel planes forming an angle of approximately 482 with the optical axis (c-axis) serve as an incident surface 81 and an exit surface, and the optical axis (c-axis) is as shown in the figure. It is arranged so that it lies on the Xz plane.

Then, y of the light beam 101 that is perpendicularly incident on the incident surface 81
The direction of polarization is that an ordinary ray passes straight through the first birefringent crystal 21 and becomes a coaxial light beam 102, whereas the X-direction polarized component of the light beam 101 is refracted at the incident surface 81 as an extraordinary ray. The light beam is separated from the ordinary ray (polarized light component in the y direction) and refracted again at the exit surface to become a light beam 202 that is parallel to and different from the coaxial light beam 102 described above. The effects of this type of birefringent crystal are detailed in Matsushita's paper ``A Configuration of an Optical Branch Circuit'' (283), included in the 1975 Proceedings of the National Conference of the Optical and Radio Division of the Institute of Electronics and Communication Engineers.

Both light beams 102, 202 then enter a Faraday rotator 22 made of praseodymium glass. A magnetic field is applied to this Faraday rotation element 22 in the z direction by an electromagnet (not shown), and the polarization direction of the light passing in the forward (z) direction is rotated by 45 degrees counterclockwise when viewed toward the light source. . The light beams 102, 202 then pass through this Faraday rotation element 22, thereby becoming light beams 103, 203 polarized in directions ±45 with respect to the X direction, as shown. Then these light beams 10
3,203 enters the optically active crystal 24. This optically active crystal 24 is made of right-handed quartz crystal and rotates the polarization direction of light passing in the direction of the optical axis (c-axis) by 45 degrees clockwise when viewed toward the light source, so that the light beam incident in the direction of the optical axis 103,203
is the incident light beam 102,2 on the Faraday rotator 22
Light beams 104 and 204 with the same polarization direction as 02, respectively.
It emits light. These light beams 104, 204 then enter a second birefringent crystal 25, which has exactly the opposite effect to the first birefringent crystal 21, and therefore in principle forms a single parallel beam without loss. It is combined into a beam 105, focused by the second convergent light transmission body 12 with a pitch of The light is transmitted to a parallel light beam 105 by the second convergent light transmitter 12 (the optical paths of the forward and reverse light beams are the same except for the last one, so if they are the same, both will be denoted by the same reference number for convenience. .

(the same applies hereafter) and enters the second birefringent crystal 25.
Since the polarization state of the light beam 105 in the opposite direction is arbitrary, this light beam 105 is generally the same as the light beam 104 polarized in the y direction.
The light beams 204 are separated into light beams 204 polarized in the and incident. The polarization directions of these light beams 103 and 203 in the opposite direction are the same as the polarization directions of the light beams 103 and 203 in the forward direction, respectively, because the second birefringence element 25 and the optically active crystal 24 are reversible.
The Faraday rotation element 22 is irreversible, and the polarization direction of light passing in the opposite direction is 45 degrees clockwise when viewed from the light source.
Since the light is rotated, the polarization directions of the light beams 102 and 202 that have passed through the Faraday rotation element 22 in the opposite direction differ from the polarization directions of the light beams 102 and 202 in the forward direction by 90 Bi, respectively. Therefore, these light beams 102,
202 enters the first birefringent crystal 21, the light beam 101 in the forward direction is separated into two light beams 102, 202. The optical path cannot be reversed, and the light beam 101 in the forward direction is separated from the light beam 101 in the forward direction. Light beams 109,2 with different optical paths
09 and will be emitted. To be specific,
The light beam 102 with polarization in the opposite direction in the x direction becomes an extraordinary ray in the first birefringent crystal 21, is refracted, and is emitted to the left (-x) side of the light beam 101 in the forward direction, and is emitted in the y direction. The light beam 202 in the opposite direction of polarization becomes an ordinary ray in the first birefringent crystal 21, so it travels straight and becomes the light beam 101 in the forward direction.
The light is emitted to the right (10x) side. These light beams 109 and 209 are parallel to the forward light beam 101 but at different positions, so they are blocked by a light shielding plate (not shown).
It does not enter the first convergent light transmission body 11. Furthermore, even if the light is incident on the first convergent light transmission body 11, it is not effectively coupled to the first optical fiber 1.

Therefore, in any case, it functions as an optical isolator.

As is clear from the above description, the optical isolator of this embodiment changes only the intensity ratio of the light beams 102, 202 (etc.) even if the polarization state changes for forward incident light; Regardless of the polarization state, it is theoretically possible to achieve no loss and actually low loss.

Furthermore, since the light beams 102 and 202 emitted from the first birefringent crystal 21 in this embodiment are parallel to each other and can be incident on the second birefringent crystal 25 to be combined, the second, Unlike the third embodiment, there is no need to bend the optical path using a reflecting mirror or the like. It has the advantage of simple configuration and no extra loss.

Furthermore, since there is no need to use a reflecting mirror or the like, the distance between these two light beams 102, 202 can be made considerably small, and the Faraday rotation element 22 and the optically active crystal 24 can be
This makes it easy to use the same beams 102 and 202 in common, resulting in advantages such as simplicity of construction, ease of arrangement adjustment and fixing, miniaturization of dimensions, and cost savings. In this first embodiment, for convenience of explanation, the spaces between each element (first focusing optical transmitter 11, first birefringent crystal 21, Faraday rotation element 22, optically active crystal 24, . . . ) are However, in order to make the whole element compact, each element may be placed in close contact with each other. Furthermore, each element may be placed in close contact with each other using a transparent adhesive or matching liquid with a birefringence similar to that of these elements (approximately 1.5). By filling in the spaces between the elements, surface reflection from each element can be almost eliminated, and by gluing them together, the positional relationship can be stabilized.

Note that an optical isolator can also be realized by omitting the optically active crystal 24 and instead changing the optical path with a reflecting mirror and tilting the second birefringent crystal 25, but the details will be explained in the third embodiment. I will discuss it after the example. FIG. 2 is a perspective view of a second embodiment of the invention.

As in the first embodiment, the light beam 101 that is emitted from the first optical fiber 1 and converted into parallel light by the first converging light transmission body 11 enters the first Rosillon prism 31, and The light beams are divided into light beams 112 and 212 with mutually orthogonal polarization. Since these two light beams are not parallel, the direction of one of the light beams 212 is changed by the first reflecting mirror 42 to make it parallel, and then the light beams are made incident on the first and second Faraday rotation elements 32 and 33, respectively. The output beams 113, 21
3 is further passed through an optically active crystal 34. The Faraday rotation elements 32, 33 and the optically active crystal 34 used in this) are the first
Since elements similar to the Faraday rotation element 22 and the optically active crystal 24 in the embodiment are functionally the same, the manner in which the polarization state changes is the same as in the first embodiment. The optically active crystal 34 emits the light beams 114 and 214, one of which is the light beam 21.
4 is changed by the second reflecting mirror 44, and then combined by the second Rosillon prism using the orthogonality of polarization, and the combined light beam 115 is generated in the same way as in the first embodiment. Second
The light is focused by a convergent optical transmission body 12 and coupled to a second optical fiber 2. Next, considering the light emitted from the second optical fiber 2 in the opposite direction, as in the first embodiment, this light follows the optical path of the forward light beam in the opposite direction and reaches the first Rosillon prism. However, Faraday rotating element 3
2 and 33 in the opposite direction, the direction of its polarization is perpendicular to that in the forward direction. Therefore, the light beam 112 traveling straight is bent by the first Rossillon prism and becomes a light beam 219. Therefore, the light beam 212 bent by the first reflecting mirror 42 travels straight and becomes the light beam 119. That is, no light beam retraces the optical path of the forwardly incident light beam to the first Rossillon prism, thus fulfilling the function of an optical isolator. In this second embodiment, the light beams 119, 219 emitted from the first rotary prism in the opposite direction
Since the direction of the light beam is different from that of the forward incident light beam 101, it has the advantage that it is more difficult to couple to the first optical fiber 1 than in the first embodiment when a light shielding plate is used. For forward-directed incident light, a low-loss device similar to the first embodiment can be obtained, but the dependence on the polarization state of the incident light is similar to that of the first embodiment.
Also, since the reflectance of the second reflecting mirror cannot be made completely 100%, a slight difference from the first embodiment usually occurs. In order to completely eliminate this dependence, it is necessary to intentionally give a small loss to the light beam 112 (etc.) traveling straight.

In this second embodiment, the first focusing optical transmission body 11
and the first Rossillon prism 31, between the Faraday rotation elements 32, 33 and the optically active crystal 34, and between the second Rossillon prism 35 and the second convergent light transmitting body 12, As in the first embodiment, it is possible to downsize it and to stabilize the positional relationship by removing surface reflection by adhering it.

Furthermore, in the first and second embodiments, one optically active crystal 24 or 34 was used for two orthogonally polarized light beams 103, 203 or 113, 213; Alternatively, two optically active crystals may be used.

In particular, the two light beams 113, 21 as in the second embodiment
This allows the optically active crystal to be smaller and therefore more economical, in cases where the spacing of 3 is relatively far apart. Also, by changing the direction of the magnetic field and reversing the direction of the Faraday rotation element, the optically active crystal 24,
A left crystal may be used as 34. Of course, other crystals and organic liquids exhibiting optical activity can also be used. This optically active crystal 24 or 34 is rotated by a Faraday rotation element 22 or 32, 33, and a forward light beam 103, 2
03 or 113, 213 is returned to its original polarization direction, which causes the subsequent two light beams 104,
204 or 114, 214 can be synthesized symmetrically with the first birefringent crystal 21 (or Rosillon prism 31).

This will become clear when compared with the third embodiment described below, but it is very useful for simplifying the configuration, and is also effective in reducing the polarization dependence of loss for forward incident light. FIG. 3 is a perspective view showing a third embodiment of the present invention, with some of the same parts as those of the second embodiment shown in FIG. 2 being omitted.

Forward light beams 113, which are emitted from the first and second Faraday rotation elements 32, 33 in the same manner as in the embodiment shown in FIG.
Since the polarization direction of the light beam 213 is tilted by ±451 with respect to the It is arranged at an angle of -45 with respect to the case of the embodiment. Therefore, the other light beam 224 that enters the second Rosillon prism 35 includes the central axis of the straight-progressing light beam 113 and the -4 direction.
5, and the direction of polarization must be perpendicular to this plane. Second, third, fourth, fifth reflecting mirrors 51, 52, 53, 54
creates a light beam 224 that satisfies these two conditions from the light beam 213 emitted from the second Faraday rotation element 33. First, the second reflecting mirror 51 converts this light beam 21
3 into a light beam 221 in the direction -45 with respect to the X direction on a plane 301 parallel to the
The fourth reflector 53 redirects the light beam 222 on this plane 301 into a light beam 223 bent by 900 degrees. Since this light beam 223 also lies on the plane 302 which forms a -45 angle with the X direction, it is incident on the second Rossillon prism 35 by the fifth reflecting mirror 54 to form a combined light beam. It becomes possible to convert to H.224. To create this final light beam 224 ignoring the polarization direction, J
Although it is sufficient to have two reflecting mirrors in this part, it is not enough to make the polarization direction perpendicular to the plane 302 which forms -45 mirrors with the X direction. In this third embodiment, the direction of polarization is changed to Xy by the second reflecting mirror 51.
A state is created in which the light lies on a plane 301 parallel to the plane, and the third and fourth reflecting mirrors 52 and 53 maintain this state, so that the light is reflected by the fourth reflecting mirror 53. The polarization direction of the light beam 223 is naturally perpendicular to the plane 302 which forms -45 bis with the x direction.
The reflecting mirror 54 obtains the light beam 224 necessary for ideal synthesis without changing its state. In this third embodiment, the number of reflecting mirrors may be reduced without using four reflecting mirrors such as the second, third, fourth, and fifth reflecting mirrors 51, 52, 53, and 54; In this case, the polarization state of the light beam 224 that obliquely enters the second Rossillon prism 35 deviates from the ideal state, causing a loss during synthesis and also deteriorating the isolation with respect to light in the opposite direction. Don't forget.

This is because the ideally polarized light beam 224 cannot be obtained with a small number of reflecting mirrors, which means that the polarization direction of the light beam 224 that is obliquely emitted from the second Rossillon prism 35 is necessarily Since it is ideal (perpendicular to the plane 302 making -450 degrees with the x direction), the polarization state when it is reflected by those mirrors and enters the second Faraday rotation element 33 from the opposite direction is as follows. forward light beam 213 of
This is because, unlike the above, this means that a component that returns to the first optical fiber 1 along the same optical path as the forward direction will eventually appear. However, this reduction in the number of reflecting mirrors is effective for applications where this reduction in isolation is acceptable. In the first embodiment as well, it is possible to do the same thing as in the third embodiment where the optically active crystal 34 in the second embodiment is replaced with reflecting mirrors 51, 52, 53, and 54. It should be obvious.

In this case, the two incident light beams to the second birefringent crystal 25 tilted by -45 may be parallel to each other, so the reflection that should be used. It is more advantageous than the third embodiment in that two mirrors are sufficient (however, the interval between the light beams incident on the second birefringent crystal 25 is equal to the output of the first birefringent crystal 21). It is desirable to change the thickness so that it is 1/V! of the interval between the emitted light beams in order to preserve the polarization state). It will also be apparent that the second Rochlon prism 35 of the third embodiment can be replaced with one of the same type as the birefringent crystal 25 of the first embodiment to reduce the number of reflecting mirrors. In the second and third embodiments, the Rosillon prisms 31 and 35 may be replaced with other types of birefringent prisms, such as Nicol prisms or Wollaston prisms.

Moreover, total reflection prisms or the like may be used instead of the reflecting mirrors 42, 44, 51, 52, 53, and 54. In addition, in the above embodiments, the pitch of the convergent optical transmitters 11 and 12 is not limited to Y, but is β5...
4.4. are also functionally almost the same.

Furthermore, these may be replaced with ordinary spherical lenses. In addition, the light does not need to pass through the optical fibers 1 and 2, and the parallel light beam 101 is incident on the first birefringent crystal 21, the first Rosillon prism 31, etc. Parallel light beam 105 from a Rossillon prism 35 etc.
, 115 is sufficient to emit light. Further, the birefringent crystals 21, 25, the Rosillon prisms 31, 35, etc. do not need to be made of calcite, and may be made of other birefringent crystals such as quartz.

As the material for the Faraday rotation elements 22, 32, 33, glass containing rare earth elements, lead, bismuth, etc. can generally be used, and for infrared light, crystals such as YlG can be used. In the embodiment, the light beam travels straight through the Faraday rotary elements 22, 32, 33, but it may also travel in a zigzag manner between two reflective surfaces, and this shape is particularly effective when permanent magnets are used instead of electromagnets. be. Note that it is also possible to use the emitted light beams 109, 209 or 119, 219 in the opposite direction without blocking them with a light shielding plate, or they can be combined and used using the orthogonality of polarization.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the most preferred first embodiment of the invention, FIG. 2 is a perspective view showing the second embodiment of the invention,
FIG. 3 is a perspective view showing characteristic parts of a third embodiment of the invention. In addition, in the figure, 1, 2...optical fibers,
11, 12... Focusing optical transmission body, 21, 25...
・Birefringent crystal, 31, 35... Rocillon prism, 22, 32, 33... Faraday rotation element, 24, 34... Optical rotation crystal, 42, 44, 5
1 to 54...Reflector, 81...Incidence surface, 1
01-105, 109, 112-115, 119, 20
2-204, 209, 212-214, 219, 221
~224... Light beam, 151... Polarization state diagram, 301, 302... Plane.

Claims (1)

[Claims] 1 A first birefringent element that splits an incident light beam into two linearly polarized light beams according to its polarization, and a polarization direction of each linearly polarized light beam, where m and n are integers, respectively (2 m ± 1/2) right angle and (2n±1/2) right angle (
a second birefringence element that combines two linearly polarized light beams that have passed through this irreversible polarization rotation element by utilizing the orthogonality of the polarizations. Including optical isolator.